Electronic driver circuits for driving an LED load often include a controllable DC-DC converter for providing a controlled current to the load. When the load contains multiple parallel strings of LEDs, the driver circuit often requires complex current regulator circuits to generate identical currents through each LED string. Generating identical LED currents is complicated by the fact that the forward voltage of the LED strings can differ.
The need for complex current regulator circuits is eliminated in driver circuits that drive only a single LED string. In this case, the DC-DC converter can provide a controlled current to the LED string based on a simple feedback arrangement, such as sensing the voltage across a sense resistor coupled in series with the load.
An external signal, as may be referred to as a pulse width modulation (PWM) signal, may be used to control the brightness of the LEDs by turning on and off the DC-DC converter with a duty cycle proportional to the desired brightness. It is desirable for the PWM signal to quickly turn on and off the DC-DC converter in order to achieve the desired brightness, as is possible when the PWM signal controls the load current with a current regulator circuit. Thus, while the elimination of costly and complex current regulator circuits in a single string LED driver is desirable, the fast load current control that is useful for PWM brightness control and that can be achieved with the use of such current regulator circuits is also lost.